1. Field of the Invention
This invention relates generally to liquid heaters and particularly to swimming pool and spa heaters, which transfer heat from products of combustion to water as it is being circulated through a heat exchanger.
2. Description of the Prior Art
Water heaters for swimming pools or spas generally comprise a burner unit, an insulated combustion chamber above the burner, and a heat exchanger above the insulated combustion chamber for facilitating heat transfer between heated air in the insulated combustion chamber and water contained in the heat exchanger. A heat exchanger header manifold is provided at one end of the heat exchanger to connect water inlet and outlet ports to a plurality of water tubes in the heat exchanger. A return header is provided at the tube ends located at another end of the heat exchanger.
Typically, the manifold header and the return manifold are made from cast iron or brass to withstand the high temperature and pressure conditions. The design of these prior art headers results in a relatively large area of surface-to-surface contact between the header and manifold and the tube supporting plate at each end of the heat exchanger. Such an arrangement is shown in U.S. Pat. No. 5,138,007. As is set forth in that patent, prior art headers reached temperatures necessitating the use of copper or other high temperature piping material for the last several feet of piping leading to the header, in place of less expensive plastic piping.
While the invention defined in the aforementioned patent permits the use of direct plastic piping connections to the manifold, the use of metallic materials for the manifold headers and return manifolds have distinct disadvantages. Those materials add considerable weight to the unit, thereby increasing handling and shipping costs. Further, as noted above, the relatively large area of surface-to-surface contact between the tube supporting plates and the headers causes the transfer of heat to the headers. Relatively cool water flowing through the manifold header results in condensation which, when mixed with low pH combustion gases, can be corrosive to the metal structure of the heating unit and other internal components.
Attempts have been made to overcome these problems by fabricating the manifold header from plastic to reduce manufacturing and shipping costs. The use of such headers has resulted in leaks and condensation within the heat exchanger. Even high temperature plastics may not be able to withstand the temperatures generated on internal components if large surface areas of the headers are in surface-to-surface contact with the tube supporting plate.